1. Field of the Invention
This invention relates to in situ recovery of shale oil, and more particularly to techniques for forming a row of in situ oil shale retorts, which can be operated as a module.
2. Descriptiposits. The term "oil shale" as used in the industry is in fact a misnomer; oil shale is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; and 4,238,136, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the fragmented mass. In the combustion zone, oxygen from a retort inlet mixture is depleted by reaction with hot carbonaceous material in the oil shale to produce heat, combustion gas and combusted oil shale. By continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, and a residual solid carbonaceous material. The residual carbonaceous material provides most of the fuel for the combustion zone as it advances through the fragmented mass.
The liquid and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process.
An in situ oil shale retort can be formed by excavating one or more generally horizontally extending voids within the boundaries of a retort site. One or more zones of unfragmented formation remain in the retort site, each with a horizontally extending free face adjacent to such a void. Explosive charges are placed in such a remaining zone of unfragmented formation for explosively expanding the formation towards such a void. This forms fragmented permeable mass of formation particles in the retort. According to some techniques for forming an in situ oil shale retort, such a void in the retort site can be thirty to sixty or more feet high. Such voids can be 100 to 200 feet across or in some cases, even longer in one direction. During development of a large oil shale tract dozens of such voids can be in various stages of excavation at a given time. It is desirable to provide fast and economical techniques for excavating such large voids.
It is sometimes desirable to form in situ oil shale retorts in one or more rowe to provide modules of retorts that can be processed at substantially the same time. By operating retorts in modules or groups, economical control can be facilitated. Liquid and gaseous products withdrawn from all of the retorts in the module can be commingled thereby minimizing separate facilities needed for handling the products of retorting. Entire modules of retorts rather than individual retorts can be isolated from other underground workings for avoiding spread of noxious or toxic gases through such underground workings.
U.S. Pat. No. 4,106,814 describes a technique for forming a row of horizontally spaced apart in situ oil shale retorts. Retort access cross drifts are excavated through a plurality of retorts in such a row. Each retort is formed by excavating horizontal voids at the elevations of the retort access drifts. Each retort is then formed by explosively expanding formation within the boundaries of such a retort site towards such voids for forming a fragmented permeable mass of formation particles in each retort. In one embodiment described in this patent, the cross drifts through the retorts are about 20 feet high and the corresponding horizontal voids are excavated to about the same height. Since some embodiments of techniques for forming in situ oil shale retorts employ considerably higher voids, it is desirable to provide techniques for economically excavating such high voids in a row or module of retorts.